The conventional heat-shrinkable films, particularly such as used for labels to be wrapped around the bodies of bottles, have been made mainly of polyvinyl chloride, polyethylene or other polymers. In recent years, however, it has been pointed out that these polymers have some drawbacks; for example, polyvinyl chloride evolves chlorine-containing gases in the burning for waste disposal and polyethylene is difficult to effect the printing of labels. Furthermore, when polyethylene terephthalate (PET) bottles having the labeling printed on the heat-shrinkable films are recovered for recycle use, such labels as made of resins other than PET should be separated from the PET bottles. For these reasons, heat-shrinkable polyester films have attracted much attention.
Most of the conventional heat-shrinkable polyester films, however, suddenly shrink by heat treatment, and they have wrinkles, shrinkage spots and strains remaining after shrinkage; therefore, they cannot be considered satisfactory shrinkable films for label applications.
To solve some of these defects, JP-A 1-110931/1989 discloses a method for improving the finish of shrinkage of polyester films by remarkably reducing their elongation at break in the direction perpendicular to the major shrinkage.
The polyester films obtained by this method, however, have poor utility as shrinkable films for label applications because not only do they easily break under tension applied in the printing, labeling or other steps but also the rupture of labels easily occurs during the ordinary transport of bottles as filled with contents.
JP-A 63-309424/1985 discloses a method for obtaining heat-shrinkable polyester films with improved finish of shrinkage by controlling their shrinkable properties at 75.degree. C.
JP-A 4-170437/1992 discloses a heat-shrinkable polyester film formed from a polymer blend of polybutylene terephthalate and polyethylene naphthalate.
The polyester films obtained by these methods are, however, still unsatisfactory as shrinkable films for label applications. PET bottles, which are mainly used for beverage vessels in recent years, have been rapidly changed from the conventional round type to the square type in view of their storage in the refrigerator as disclosed in JP-A 63-309424/1988 and JP-A 4-170437/1992. When tube-shaped labels made of heat-shrinkable polyester films are fitted around the bodies of square bottles, such a defect easily occurs that the films on the flat portions other than on the corners of the bottles are curved in arch shape after shrinkage (herein referred to as "longitudinal sinking"). Taking into consideration the fact that square PET bottles are being mainly used for beverage vessels, heat-shrinkable polyester films scarcely causing longitudinal sinking when used as display labels for such bottles should be developed.
In addition to these favorable properties for label applications, they are further required to have high transparency, better color development of ink and improved finish of shrinkage, i.e., very few wrinkles, shrinkage spots and strains remaining after shrinkage.